Vehicle brake system having an anti-skid apparatus

ABSTRACT

The proposed vehicle brake system includes a brake fluid pressure device embodied as a brake booster, front wheel brakes rear wheel brakes and an anti-skid apparatus having valves for maintaining, reducing and increasing brake pressure. One of the brake pressure maintenance valves is associated with the rear wheel brakes and is further developed into a valve combination, which additionally includes a brake pressure regulating valve. The valve combination includes only one valve seat and one valve element, which serves both for keeping braking pressure constant in the anti-skid situation and to limit the rear wheel braking pressures during braking. The valve element is controlled on one side by a control piston acted upon by brake pressure and on the other by an electromagnet.

BACKGROUND OF THE INVENTION

The invention is based on a vehicle brake system as generally definedhereinafter.

In order to attain the shortest possible vehicle stopping distances of avehicle in its track without undue swerving, it is known to incorporatebrake pressure regulating valves in the brake lines leading to the rearwheel brakes (U.S. Pat. No. 2,991,797 and German Offenlegungsschrift 3222 798). When wheel stresses have been dynamically altered because ofbraking deceleration, these brake pressure regulating valves avoidexcessively severe actuation of the rear wheel brakes. The brakepressure regulating valves are preferably installed in vehicles having afront wheel drive. Depending on the type of brake pressure regulatingvalve used, the theoretically maximum possible rear wheel braking forcesare only partially attained.

Anti-skid brake systems are shown in U.S. Pat. No. 4,478,461 and4,484,784 which include control for controlling the front and backbrakes. Prior art systems have the advantage that with increasinglyforceful pedal actuation, they limit braking pressures whenever themaximum possible friction between the wheel and the road surface at agiven time has been attained. If a severely braked vehicle moves from agripping surface to one that has less grip, such as ice, then in suchanti-skid brake systems the braking pressures are reduced until suchtime as the maximum possible friction between wheel and road surface hasagain been attained. The known brake pressure regulating valves for rearwheel brakes can also be built into anti-skid brake systems, for thesake of their known advantages. However, building in a brake pressureregulating valve of simple embodiment may have the disadvantage, if therear wheel brake pressure limitation has attained a maximum value and atleast one front wheel of the vehicle is tending to skid, that themaximum possible friction between the rear wheels and the road surfacecannot be exploited for decelerating the vehicle. In a particular typeof anti-skid system, the driver is warned of tendencies toward skiddingby means of vibrations at the pedal of the vehicle brake system. Withanxious drivers, the result can be that even in an emergency, where themaximum possible braking deceleration is essential, they will notactuate the pedal as severely as the situation warrants. Building inknown brake pressure regulating valves has the further disadvantage thatit makes the vehicle brake system more expensive, and it increases thenumber of locations at which leakage losses can occur.

OBJECT AND SUMMARY OF THE INVENTION

The method according to the invention has the advantage that the maximumpossible friction between the rear wheels and the road surface at aparticular time can be exploited. Further embodying the method inaccordance with this teaching has the advantage that even in the eventof brake circuit failure, which normally results in a shutoff of theanti-skid apparatus, forceful emergency braking is still attainable. Themethod having the characteristic as taught herein has the advantage thateven if one brake circuit in a multiple-circuit system should fail,forceful braking is still possible.

By means of the provisions revealed advantageous further embodiments ofand improvements to the vehicle brake system according to the inventionare attainable. This application further recites characteristics bymeans of which the cost of manufacture of the system and of installingit in a vehicle is lower than when previously known conventional valvesseparate from one another are used. Additionally, because there arefewer connections, there is greater protection against leakage losses.The concept according to the invention, that is, to unite a brakepressure regulating valve with a brake pressure maintenance valve, canalso be realized without including the bypass valve.

The characteristics disclosed herein serve particularly to perform themethod and a corresponding structure has the advantage already mentionedthat in the event of leakage in a front wheel brake circuit, at leastone rear wheel brake continues to be acted upon with unreduced pressurefrom the brake pressure provider. This compensates at least partiallyfor losses in front wheel braking force and thereby increases drivingsafety.

The invention will be better understood and further objects andadvantages thereof will become more apparent from the ensuing detaileddescription of preferred embodiments, taken in conjunction with thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a first exemplary embodiment of the vehicle brake systemaccording to the invention, having the valve combination;

FIG. 2 is a diagram characterizing a rear wheel brake pressurelimitation for the first exemplary embodiment;

FIG. 3 shows a second exemplary embodiment of the invention; and

FIG. 4 shows a further development of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The first exemplary embodiment of a vehicle brake system shown in FIG. 1illustrates a device for developing brake pressure in the form of aknown footoperated brake booster 2, an anti-skid apparatus 3 such asshown in U.S. Pat. No. 4,484,784 and in U.S. Pat. application Ser. No.675,043 filed Nov. 23, 1984 having a valve block 4, front wheel brakes5, 6, rear wheel brakes 7, 8 and a valve combination 9 which functionsin cooperation with the rear brakes.

The brake booster 2 corresponds by way of example to the type describedin U.S. Pat. application Ser. No. 675,043 filed Nov. 23, 1984. It iscontrolled by means of a pedal 10. The valve block 4 contains anti-skidvalves, not shown, the operation of which is also described in U.S. Pat.application Ser. No. 675,043 filed Nov. 23, 1984. The brakes 5, 6, 7, 8can also be taken from the prior art and are therefore not described indetail herein.

The valve combination 9 includes a brake pressure maintenance valve 11and a brake pressure regulating valve 12. The valve combination 9 has avalve chamber 13, located adjacent a valve seat 14; a movable valveelement 15; an adjusting cylinder 16; an end closure 16a; an axial stop17 in cylinder 16; an adjusting piston 18, and a pressure limitingspring 19 in cylinder 16; an opening spring 20; an opening tappet 21;and a closing spring 22. The adjusting cylinder 16 merges with the valveseat 14 in the projection of the longitudinal axis of the valve chamber13. The axial stop 17, which is embodied in the form of a ring, ispressed into the adjusting cylinder 16 adjacent the valve seat 14. Aradially oriented aperture 23 is machined into the axial stop. Inalignment with this recess 23, the adjusting cylinder 16 has a rearwheel brake line connection 24. Near the valve seat 14, the valvechamber 13 has a brake fluid pressure line connection 25. The brakebooster 2 serves as a brake fluid pressure applicator and is connectedwith the brake fluid pressure line connection 25. The adjusting piston18 is displaceably built into the adjusting cylinder 16 next to theaxial stop 17, and it is sealed off with respect to the inner wall ofthe adjusting cylinder 16 by means of an O-ring seal 26. The pressurelimiting spring 19 is oriented between the end closure and the adjustingpiston 18 with the piston located between the spring 19 and the axialstop 17. The adjusting piston 18 has a coaxially aligned springreceiving chamber 27, which is open toward the valve seat 14 and whichreceives opening spring 20. The opening tappet 21 carries a spring plate28, which is displaceable inside the spring receiving chamber 27 counterto the force of the opening spring 20 and which is prevented fromdisplacement out of chamber 27 by protrusions 40. Thus, beginning at theadjusting piston 18, the opening tappet 21 extends as far as theinterior of the valve chamber 13. The closing spring 22 is supportedinside the valve chamber 13 by means of protrusions 29 in such a mannerthat it urges the movable valve element 15, which is embodied forinstance in the form of a ball, in the direction toward the valve seat14.

The brake pressure maintenance valve 11 shares the valve chamber 13, thevalve seat 14 and the valve element 15 in common with the brake pressureregulating valve 12 and also has an electromagnet 30 having a magneticcoil 31 and an armature 32. The armature 32 is aligned coaxially withthe adjusting cylinder 16 and has a closing tappet 33, which is orientedin the direction of the valve seat 14 and rests against the valveelement 15.

The vehicle brake system functions as follows:

When the pedal 10 is actuated, the brake booster generates brakingpressure, which travels through the valve block 4 and lines 34, 35 tothe front wheel brakes 5 and 6 and via a line 36 to the brake fluidpressure line connection 25 of the valve combination 9. If the fluidpressure generated by the brake booster 2 by means of the actuation ofthe pedal 10 is lower than a preselected pressure threshold, then thevalve element 15 remains in the open position shown in FIG. 1, in whichit is spaced from the valve seat 14, thereby opening the valve, for thelow pressure is incapable of acting upon the adjusting piston 18 soforcefully that the adjusting piston 18 is displaced away from its axialstop 17 counter to the action of the pressure limiting spring 19. As aresult, the pressure limiting spring 19 compresses the closing spring22, via the adjusting piston 18, the opening spring 20 and the openingtappet 21; thus the valve seat 14 remains opened. The open valve seat 14causes the valve chamber 13 to communicate with the adjusting cylinder16 and the rear wheel brake via line 37 connected to connection 24. As aresult, the braking pressure prevailing in the line 36 is carried on tothe rear wheel brake connection 24 and from there to the rear wheelbrakes 7 and 8, via lines 37, 38 and 39. Accordingly, in the front wheelbrakes 5 and 6 and in the rear wheel brakes 7 and 8 braking pressuresrise in accordance with how they are generated by the brake booster 2.For instance, if main cylinders built into the brake booster 2 have thesame diameter for both the front wheel brakes and the rear wheel brakes,then the braking pressures in the front wheel brakes and in the rearwheel brakes rise to the same pressure levels.

If the actuating force upon the pedal 10 is increased, then the pressureexerted on the adjusting piston 18 increases as well. Finally, thepressure threshold mentioned above in cylinder 16 will be exceeded,causing the force of the pressure limiting spring 19 to be overcome. Asa result, the adjusting piston 18 moves away from the axial stop 17 andthereby, via coupler protrusions 40, moves the spring plate 28 and theopening tappet 21 connected to the spring plate 28 away as well.Consequently, the closing spring 22 moves the valve element 15 againstthe valve seat 14, and no further pressure medium flows to the rearwheel brakes 7 and 8; as a result, no further pressure increase takesplace in these rear wheel brakes. Meanwhile, by means of a still moreforceful actuation of the pedal 10, the braking pressure in the frontwheel brakes 5 and 6 can rise still further. This is shown in the formof a graph in FIG. 2, in which PV stands for the braking pressures inthe front wheel brakes 5, 6 or at the brake fluid pressure lineconnection 25, and PH stands for brake pressures in the rear wheelbrakes 7 and 8. A straight line A originating at coordinate point 0 andrising from there indicates that when the pedal 10 is actuated, thepressures PV and PH initially rise. Once the brake pressure PV attainsthe abovementioned pressure threshold PVI of the brake pressureregulating valve 12, any further rise of the braking pressure PH isprevented, as represented by the horizontally extending line B in thediagram. If the actuation force at the pedal 10 is lessened, then thefront wheel brake pressure PV decreases, and when it passes below thepressure threshold PV1, the pressure limiting spring 19 presses themovable valve element 15 away from the valve seat 14, via the adjustingpiston 18, the opening spring 20 and the opening tappet 21. As a result,the valve combination 9 is opened, and if the front wheel brake pressurePV is reduced further by means of the pedal 10, then the rear wheelbrake pressure PH drops as well, along the sloping straight line A.

If the electromagnet 30 is switched on, then the armature 32 presses thevalve element 15, via the tappet 33, against the opening tappet 31 andthereby overcomes the force of the opening spring 20 which permits valveelement 15 to move toward cylinder 16, so that the valve element 15rests tightly against the valve seat 14. The electromagnet 30 isembodied strong enough, for example, that the valve element 15 remainsat the valve seat 14 as long as a predetermined pressure drop betweenthe rear wheel brake connection 24 and the brake fluid pressure lineconnection 25 is not exceeded. As a result, the valve combination 9 iscapable of keeping brake pressure in the rear wheel brakes 7, 8 at thesame level, while a pressure in the line 36 is reduced. The latter mayhappen, for instance, as a result of triggering the valve block 4. Thevalve block 4 is triggered if for instance one of the front wheel brakes5, 6 brakes a wheel associated with it so severely that there is adanger of skidding. To overcome the danger of skidding, the brakepressure must in fact be reduced. If the brake booster 2 and the valveblock 4 have typical characteristics as described in German patentapplication P 34 09 290.0, which is the same as U.S. Pat. applicationSer. No. 675,043 filed Nov. 23, 1984 now U.S. Pat. No. 4,624,108, thisbrings about the above-mentioned reduction of the pressure in the line36. As long as the wheels braked by the rear wheel brakes 7, 8 are notsubject to a danger of skidding, it is disadvantageous to reduce thebrake pressure in these brakes. This undesirable pressure reduction isprevented by switching on the electromagnet 30 as described.

It should also be noted that instead of the embodiment of the vehiclebrake system as shown in FIG. 1, each rear wheel brake 7, 8 may have itsown valve combination 9. The result is the known advantage that eachrear wheel brake then contributes, independently of each other, to amaximum possible braking deceleration in the most favorable manner.

The second exemplary embodiment shown in FIG. 3 differs from the firstin that its valve combination 9' additionally has a controllable bypassvalve 41. This valve has a valve chamber 42, a valve seat 43, a movablevalve element 44 which may for instance be in the form of a ball, and aclosing spring 45. The valve seat 43 communicates via a conduit 47 whichsurrounds opening tappet 46 with the valve chamber 13. The valve seat 43adjoins the conduit 47. Adjoining valve seat 43 in turn is the valvechamber 42 of the bypass valve 41, which opens into the recess 23 of theaxial stop 17. The closing spring 45 is inserted into the valve chamber42 between the axial stop 17 and the valve element 44 and is strongenough that the valve element 44 also remains on its valve seat 43 if,with the valve seat 14 properly sealed, a predetermined pressure drop isnot exceeded between the brake fluid pressure line connection 25 and therear wheel brake connection 24. An opening tappet 46 is rigidly securedto an armature 32' and oriented toward the movable valve element 44.This armature 32' differs from the armature 32 of the first exemplaryembodiment in that it has a closing spring receiving chamber 48, whichis disposed concentrically and opens in the direction of the valveelement 15. The closing spring receiving chamber 48 receives a closingspring 49 and a spring plate 50. The spring plate 50 is joined to aclosing tappet 33' oriented toward the valve element 15. Protrusions 51disposed on the armature 32' assure that the spring plate 50 and theclosing spring 49 remain within the closing spring receiving chamber 48.The part of the valve combination 9' that embodies the magneticallycontrollable brake pressure maintenance valve 11' is by reason of itsstructure a so-called 3-position valve, which beginning at its outsetposition, shown, is selectively controllable into a second and a thirdposition by means of two currents of different intensities, as a resultof an actuation of the magnet. In the first actuation at a lessercurrent, the armature 32' moves in the direction of the valve seat 14and by means of the closing spring 49 presses the valve element 15against the valve seat 14. The opening tappet 46 is dimensioned suchthat it reaches the valve element 44 of the bypass valve 41 no earlierthan when the valve element 15 is resting on the valve seat 14. Asalready mentioned, the closing spring 45 of the bypass valve 41 isembodied as a strong spring and therefore, at the first actuation of themagnetic coil 31 with a low current, keeps the valve body 44 against thevalve seat 43 despite the opening tappet 46 resting against it. Anactuation of the magnetic coil 31 with a second, stronger current causesthe armature 32' via the opening tappet 46 to overcome the force of theclosing spring 45 of the bypass valve 41 as well. As a result, the valveelement 44 is pressed away from the valve seat 43, and the connections24 and 25 communicate with one another by means of the valve chamber 13and the valve seat 43. In this switching position of the valvecombination 9', pressure from the brake booster 2 is delivered withoutbeing reduced, that is, at full strength, to the rear wheel brakes 7, 8.The advantage is as described in the background section above, that inthe event that one of the front wheel brakes should skid and the brakingpressure should be reduced there, a braking pressure which is at thesame level as that furnished by the brake booster 2 will still befurnished to the rear wheel brakes 7, 8. In the desired manner, thisincreases the rear wheel braking forces. By actuating the pedal 10 morestrongly, the pressure in the rear wheel brakes can be increased stillfurther. The antiskid apparatus 3 assures that if the pedal 10 isactuated too hard, the maximum possible friction between the rear wheelsand the road surface will be exploited.

This second valve combination 9', like the first valve combination 9,can also be disposed such that each one of the rear wheel brakes has itsown pressure line.

The valve combinations 9, 9' can also be used in brake systems the brakeboosters and anti-skid concepts differ from those of U.S. Pat.application Ser. Number 675,043 filed Nov. 23, 1984, such as in U.S.Pat. Nos. 4,478,461 and 4,484,784.

The exemplary embodiment shown in FIG. 4 differs from theabove-described exemplary embodiments in having a hydraulic controldevice 52, which is mounted on the brake pressure regulating valve 12.The same reference numerals as those above are used for parts whichremain the same and have the same function. The control device 52 has astepped cylinder 53, a differential piston 54, two sealing rings 55, 56and a tappet 57.

The stepped cylinder 53 has a first bore 58, the diameter of which isfor instance larger than that of the adjusting piston 18, and an endwall 59 having a fluid pressure connection bore 60. The first bore 58 isadjoined in a coaxial orientation by a second bore 61, the diameter ofwhich is larger than that of the first bore 58. A centering extension 62which is formed on the adjusting cylinder 16 plunges into the secondbore 61. Adjoining the centering extension 62, the adjusting cylinder 16has an annular flange 63. There is also an end wall 64 disposed on theadjusting cylinder 16. The centering extension 62 is aligned coaxiallywith a bore 65 penetrating the end wall 64. The tappet 57 that is joinedto the differential piston 54 extends through the bore 65, is surroundedby a sealing ring 66 inserted into the end wall 64 and is orientedtoward the adjusting piston 18. Inside the bore 61, the differentialpiston 54 has a piston section 67 having a circumferential groove 68,into which the sealing ring 55 is placed. The piston section 67 isadjoined by a piston section 69, which plunges into the smaller bore 58and is surrounded there by the sealing ring 56. The piston section 69has an impingement surface 69a, which is determined by the diameter ofthe piston section 69. The impingement surface 67a of the piston section67 is larger than the impingement surface 69a. In addition to thecentering extension 62, a connection bore 70 discharges into the bore61. The connection bore 70 communicates with a brake circuit I, of whichtwo front wheel brakes 5, 6, for instance, are a part, as shown inFIG. 1. The connection bore 60 communicates with a brake circuit II, towhich the brake pressure regulating valve 12 and at least one rear wheelbrake 7, 8 (see FIG. 1) belong. A brake fluid pressure means supplyingthe brake circuits I, II is embodied for mulitiple circuits, forinstance as a brake booster 2 (see FIG. 1).

The third exemplary embodiment as shown in FIG. 4 operates as follows:

If the vehicle brake system is properly sealed and the brake booster isproviding identical pressures to the brake circuits I and II, thenduring braking the action exerted upon the impingement surface 67apredominates over that of the impingement surface 69a. As a result, thedifferential piston 54 assumes a position in which the piston section 67rests at a transition 58a which is located between the bore 61 and thebore 58.

The tappet 57 is now spaced apart from the adjusting piston 18 such thatthe adjusting piston 18 can compress the pressure limiting spring 19. Ifthere is insufficient braking pressure as a result of a leak in thebrake circuit I, then braking pressure introduced into the bore 58 fromthe brake circuit II displaces the differential piston 54 and the tappet57 against the adjusting piston 18, because as already noted thediameter of the piston section 69 is larger than that of the adjustingpiston 18, and presses the adjusting piston 18, if it is spaced apartform the axial stop 17, reliably back toward the axial stop 17 into itsoutset position, shown. As a result, as already described, the valveelement 15 is raised away from the valve seat 14 via the opening tappet21. If there is no pressure in the brake circuit I, or if the pressurethere is dropping impermissively, the brake pressure regulating valve 12thus remains open or is opened, so that braking pressure introduced intothe brake pressure regulating valve 12 is effective, without beingreduced, in the at least one rear wheel brake via the lines 37, 38, 39.As a result, front wheel braking force that has been lost is at leastpartially replaced with increased rear wheel braking force.

The disposition of the control device 52 is not limited to the brakecircuit arrangement shown in FIG. 4. The control device 52 can also becombined with the exemplary embodiment shown in FIG. 1.

The foregoing relates to preferred exemplary embodiments of theinvention, it being understood that other variants and embodimentsthereof are possible within the spirit and scope of the invention, thelatter being defined by the appended claims.

What is claimed and desired to be secured by letters patent of theUnited States is:
 1. A vehicle brake system comprising a brake fluidpressure means for applying a brake pressure in said brake system, frontwheel brakes, rear wheel brakes, and an antiskid apparatus between saidbrake fluid pressure means which controls fluid flow from mastercylinder to said rear wheel brakes, a brake pressure regulating valveand one brake pressure maintenance valve disposed between said brakefluid pressure means and the rear wheel brakes, said brake pressureregulating valve and said brake pressure maintenance valve are unitedinto a valve combination and have a hydraulically self-controlling rearwheel brake pressure limiter, a first valve seat and a first movablevalve element, said first movable valve element is actuatable on one endby an adjusting piston and on the other end by an electromagnet of saidbrake pressure maintenance valve.
 2. A vehicle brake system as definedby claim 1, characterized in that said first movable valve element islocated inside a first valve chamber of said valve combination whichincludes a first valve seat and a brake fluid pressure means connectionin said valve combination which connects with said brake fluid pressuremeans, said adjusting piston is in longitudinal axial alignment withsaid first valve chamber, in an adjusting cylinder which includes anaxial stop and a rear wheel brake connection disposed on the axial stop,and connected with said rear wheel brakes, a pressure limiting springdisposed in said adjusting cylinder such that said limiting spring urgessaid adjusting piston in a direction of said first valve seat towardsaid axial stop, an opening spring disposed between said adjustingpiston and said first valve element, and said electromagnet is orientedtoward a closing tappet, which tappet points in a direction of saidadjusting cylinder and is thereby oriented in turn toward said firstvalve element.
 3. A vehicle brake system as defined by claim 2, in whichsaid bypass valve includes a bypass valve chamber, which is orientedparallel to said first bypass valve chamber of the valve combination,said bypass valve chamber communicates with the rear wheel brakeconnection and inlcudes therein a first closing spring, a second movablevalve element and terminates at a second valve seat which communicateswith said brake fluid pressure means connection, and said electromagnetwhich includes an armature that engages an opening tappet, which isoriented toward said second movable valve element of the bypass valve,and also, via a second closing spring, engages the closing tappet of thebrake pressure maintenance valve, said first closing spring is disposedin said bypass valve chamber of said bypass valve and is substantiallystronger than said second closing spring which engages the closingtappet, and that said electromagnet, in a first excitation stage, isexcitable in such a manner that, by means of said armature via thesecond closing spring and the closing tappet, said electromagnet pressessaid first valve element against said first valve seat, whereupon theopening tappet associated with the bypass valve comes to rest on thesecond valve element of said bypass valve, and said electromagnet, in asecond excitation stage, is excitable so that said armature raises saidsecond valve element of the bypass valve away from the second valveseat, in order to open said bypass valve.
 4. A vehicle brake system asdefined by claim 3, in which the brake fluid pressure means includesoutlets that connects to at least two brake circuits (I, II), and ahydraulic control device connected to said outlets of the brake fluidpressure means whereby if a brake circuit (I) including at least onefront wheel brake fails said brake pressure regulating valve controlsone other brake circuit (II) in such a manner that no brake pressurelimitation takes place.
 5. A vehicle brake system as defined by claim 2,in which the brake fluid pressure means includes outlets that connectsto at least two brake circuits (I, II), and a hydraulic control deviceconnected to said outlets of the brake fluid pressure means whereby if abrake circuit (I) including at least one front wheel brake fails saidbrake pressure regulating valve controls one other brake circuit (II) insuch a manner that no brake pressure limitation takes place.